Process of producing metallic disulphides, tellurides, and selenides



Patented Jan, 23, 1945 UNITED STATES PATENT OFFICE PROCESS OF PRODUCINGMETALLIC DI- SULPHIDES, TELLURIDES, AND SELE- NIDES Edwin B. Kaercher,Lima, Ohio, assignor to Westinghouse Electric & Manufacturing Company,East Pittsburgh, Pa., a corporation of Pennsyl- This invention relatesto a method and the product produced thereby, of treating certaincommercially available inorganic compounds in order to render thesecompounds suitable for use as solid lubricants.

This invention relates to the processing of metallic compounds of thetype exemplified by molybdenum disulphide, tungsten disulphide,molybdenum selenide, molybdenum telluride and tungsten telluride, andsimilar inorganic compounds of metals and non-metals characterized byflat crystals easily cleaving into thin plates. The compounds arebelieved to form crystals in which the metallic atoms are attached toone another in a single plane while two non-metallic atoms, such assulphur, are attached to each metallic atom to form layers on both sidesof the plane.

These compounds are believed to be highly effective lubricants due tothe fact that non-metallic atoms in each of the several crystals havevery little attraction for one another and accordingly slippage of thecrystals with a low coefilcient of friction occurs when the crystals aresubjected to shear stresses under load. On the other hand, thenon-metallic atoms have an affinity for metal surfaces such that underappropriate conditions the non-metallic atoms will bond to the metalsurfaces so tenaciously that the crystals may not be removed except byabrading the metal surfaces. In the specification reference will be madespecificall to the use of molybdenum disulphide, but it is intended thatany of the compounds of this class be included since they may besimilarly treated to produce equivalent results.

The. solid compounds such as molybdenum disulphide are exceptionallygood lubricants for many purposes. For certain applications molybdenumdisulphide type of compounds is the only satisfactory known materialcapable of functioning effectively as lubricants. When subjected torather high temperatures, as up to 500 C. in air, and even higher underreduced pressures, molybdenum disulphide applied properly will functionwith a high degree of effectiveness whereas most known lubricants, solidor liquid, have been found to be totally ineffective or even harmful insome cases. For example, in rotating anode X-ray tubes, the use ofliquid lubricants is impractical since the vapor pressure is sufiicientto render the X-ray tube inoperative. Customary solid lubricants, suchfor example, as graphite,

have been found to be not only ineffective as lubricants, but in someinstances have resulted in an accelerated deterioration of the bearingsof the rotating anode due to molecular re-arrangement and/ordecomposition. Furthermore, the molybdenum disulphide has been found tobe operative under extreme pressures with the lowest coefficient offriction of many commonly known solid or liquid materials. For example,at pressures of 500,000 and 600,000 pounds per square inch, thecoefficient of friction was less than one-third of several liquidlubricating substances. The coefllcient of friction of molybdenumdisulphide was many times less than that of solid lubricants which havebeen employed for some purposes. At the other extreme, at low pressures,the anti-friction properties of solid lubricants exemplified bymolybdenum disulphide have been better, for instance, with respect tothe coefficient of friction than many liquid and solid libricatingsubstances.

In order to effectively apply molybdenum disulphide to bearings, it hasbeen found that commercially available modybdenum disulphide. isgenerally unsuitable for the purpose and must be processed in order torender it suitable as a solid lubricant. Heretofore, for lubricatingprocesses it was necessary to select large, extremely pure crystals orlumps of molybdenum sulphide and employ only such material forlubricating purposes. Crystals or lumps of this kind are not availableon a commercial scale.

Commercially available molybdenum sulphide is produced by grindingmolybdenite ore and separating the molybdenum disulphide particles fromthe gangue usually by flotation. While the separation removes a greatamount of the impurities from the concentrate, the concentrate is notsatis factory as a lubricant. A relatively great amount of impuritiesand abrasive particles is present therein rendering its application as alubricant impractical.

For lubricating purposes, molybdenum disulphide must be substantiallycompletely free of abrasive material and other impurities, such assulphur, other sulphides and polysulphides of molybdenum which mayprevent the adherence of a film of molybdenum disulphide to metalsurfaces. It will be appreciated that a single minute particle of quartzin even a small amount of molybdenum disulphide, if applied to the pivotof an instrument, would easily ruin the pivot. No amount of molybdenumdisulphide will satisfactorily lubricate the pivot under theseconditions and enable it to function properly. The abrasive particle mayscore the pivot point, thus removing the adherent molybdenum filmsulphide as well as destroying the shape of the pivot itself. Therefore,unless molybdenum disulphide is produced substantially free fromimpurities and abrasives or all l-zinds, the material is not suitablefor lubricating purposes.

It should be noted that o! the numerous sulphides of molybdenum, onlythe disulphide, Moss, has any appreciable lubricating p p Henceparticular care must be taken to remove the tree sulphur andpolysulphides which occur naturally in molybdenite ore and itsconcentrates.

The object oi this invention is to provide for treating solid compoundssuch as molybdenum disulphide to render them suitable for use as solidlubricants.

Qther objects of the invention will, in part, be obvious, and will, inpart, appear hereinafter.

For the practice of this invention, raw molybdenum disulphide, such asthe concentrates produced ordinarily by flotation of finely groundmolybdenite ore, are available to the trade. The concentrates are in afinely divided state of the order of from 100 to 300 mesh. In some caseseven selectedlarge crystals or lumps of relatively pure molybdenumdisulphide may be subjected to the purification treatment hereindisclosed in order to completely eliminate impurities and abrasivestherein. The concentrates generally consist of from about 80% to 90%molybdenum disulphide, approximately 6% iron oxides and sulphides andseveral percent of quartz. The concentrates will also contain molybdenumpolysulphides, molybdenum oxides, and free sulphur; silicates and oxidesof calcium, aluminum, magnesium and copper, as well as copper and othersulphides. Since the concentrates are produced by flotation, they arecontaminated with grease, oils and flotation agents. All of theseimpurities and abrasives are generally detrimental to the functioning ofthe molybdenum disulphide as a lubricant.

In purifying the molybdenum disulphide to render it suitable as a solidlubricant, the finely pulverized sulphide is initially extracted with asolvent for the greases, oils, and flotation agents. Some extractionagents will also remove any free sulphur present in the concentrate. Asuitable extraction agent for this purpose is trichlorethylene. Otherorganic solvents, such as ethyl alcohol, acetone. carbon disulphide andsimilar powerful solvents for greases, sulphur, and flotation agents maybe used for this purpose.

For the most eflicient application of the extraction agent, the ore istreated on the Soxhlet principle. placed in a receptacle at the foot ofa condensing column whereby vapors of the solvent on con- (lensing tothe liquid phase in the column run into the ore. After the ore has beencovered to a given depth by the solvent a siphon device attached to thereceptacle begins to function and substantially all of the solvent withsolute therein is rapidly removed from contact with the molybdenumdisulphide. The contaminated solventv The finely divided ore concentrateis all the impurities. The treated molybdenum disulphide is dried in anoven at a temperature sufiilcient to eliminate all of the solventtherefrom.

In order to insure the most complete removal of all impurities, thegrease and oil free molybdenum disulphide now may be advantageouslycomminuted to much smaller dimensions. The molybdenum disulphidesuspended in some of the solvent may be transferred to a ball mill orother equivalent mill. The disulphide is milled for a period of timeuntil the particles of compound average from 1 to 50 microns indimension. Several hours in a high speed ball mill using inch diametersteel balls has been found suificient for this purpose. Subsequenttreatment will remove any steel particles wih'ehimay come oil the balls.

The ball milled molybdenum disulphide is separated from the solventliquid by centrifuging and decanting the solvent. Filtering and dryingthe molybdenum disulphide prepares it for the next step.

The advantages of the finely subdivided mineral are that subsequenttreatment is expedited and a more fully purified lubricant is obtained.Silica, for example, is present not only as large particles but also isoccluded mechanically in the molybdenite crystals. Any treatment toremove silica will be greatly hindered in its efficiency if themolybdenite crystals are not broken sufflciently to expose the silicaoccluded therein.

The dried, partially purified concentrate of molybdenum disulphide istreated with from three to ten parts of 80% hydrofluoric acid for eacheight parts of the concentrate by weight. The treatment withhydrofluoric acid can be successfully performed with liquid acid ofgreater or less concentration, or with acid entirely in the gaseousphase using hydrogen fluoride gas contained in equipment resistant tothe action of the gas, and so designed as to remove the gaseous silicontetra-fluoride formed during the treatment. The main purpose of thesetreatments is to remove silicon compounds, such as silica and silicates,normally found in the molybdenum disulphide concentrates.

The use of hydrogen fluoride gas will convert the silica, or othercombined silicon present in the concentrates, to silicon tetra-fluoride,a volatile gas. In the case of the aqueous hydrofluoric acid treatment,silicon tetra-fluoride will likewise be produced; but depending upon theamount of water present, some of this gas will be hydrolyzed to formsilicic acid and fluo-silicic acid. The latter material iswater-soluble; however, the silicic acid is not water or weakalkalisoluble, and is only acid-soluble when in the colloidal state.silicic acid is, therefore, removed as later described by dehydratlng itto silica and treating the concentrates again with hydrofluoric acid.Any silicic acid formed by hydrolysis during this second treatment iskept in the colloidal state and removed by washing with hydrochloricacid.

The addition or the liquid hydrofluoric acid to the concentrates may beefl'ected in a molded phenol-aldehyde receptacle, resistant to theaction of the acid. A paste is formed by mixing the acid and themolybdenum disulphide. This paste is allowed to stand for eight totwenty-four hours, thereby insuring conversion of all silica andsilicates to silica acid, fluosilicic acid, or silicon tetra-fluoridegas; and solution or all other hydrofluoric acid soluble minerals.

At the end of the hydrofluoric acid treatment, the paste is mixed withapproximately an equal volume of water to facilitate filtering, and thenfiltered to separate the molybdenum disulphide concentrates from thefiltrate. Washing 01' the concentrates is carried out until they havebeen washed with four to eight times their own volume of water. In thefiltrates from this washing process will be found fiuosilicic acid,excess hydrofluoric acid, molybdic acid and water soluble metalfluorides.

The washed concentrates are then rendered free of polysulphides ofmolybdenum by mixing them with an equal volume of an aqueous 20% sodiumsulphide solution and allowing this mixture to stand for 4-10 hours. Atthe end of this time the mixture is filtered and then washed with hot orboiling water until a clear filtrate is obtained. Other alkali sulphidesmay be employed instead of sodium sulphide.

In many cases the molybdenum disulphide concentrates are rendered freefrom all traces of silicon compounds by one hydrofluoric acid treatment.however, for most careful purity control it is Suggested that thehydrofluoric acid treatment be repeated upon the concentrates at thispoint in the process. Before giving the concentrates this secondhydrofluoric acid treatment, they should be thoroughly dried at 110-200C. to dehydrate any silicic acid present.

Following this second treatment, the paste of acid and concentratesshould be treated with an equal volume of hydrochloric acid to convertsilicic acid, if present, to the colloidal state. Filtration of thismixture will leave all the silicic acid in the filtrate.

The resulting molybdenum disulphide concentrates are then mixed withapproximately twice their volume of 10% hydrochloric acid, and allowedto stand for 3-4 hours in order to be certain to dissolve any coppersulphides and other acid soluble matter still present in theconcentrates. At the end of this time, the mixture should be filteredand washed with several times its volume of distilled water and finallywith distilled water made slightly alkaline by addition of ammoniumhydroxide.

The molybdenum disulphide is finally washed with boiling distilled wateruntil free of all soluble material. Completeness of washing can beconveniently determined by making conductivity measurements on samplesof the filtrate as washing proceeds. When washing is complete, theconductivity will reach a low but constant value due to the very slightsolubility of molybdenum disulphide.

For storage purposes it is generally found convenient to dry the Washedand purified molybdenum disulphide at l25200 C. The product is a dry,shiny-black, finely divided powder.

The purified powder consists of essentially pure molybdenum disulphidefree from abrasives and greases and other harmful matter whereby theconcentrated sulphuric acid, nitric acid, or other powerful oxidizingagents.

A similar procedure may be used in treating tungsten disulphide,tungsten selenide and related compounds.

The process described is designed to insure consistent and maximumpurity of the finished product, and may be considerably shortened,operations omitted etc., when ore concentrates from some sources areused, or where maximum purity is not essential.

Since certain changes in carrying out the above process may be madewithout departing from its scope, it is intended that all mattercontained in the above description shall be interpreted as illustrativeand not in a limiting sense.

I claim as my invention:

1. The process of producing metallic disulphides, tellurides andselenides'having solid lubricant properties from finely divided rawmaterials containing said compounds which comprises subjecting saidfinely divided raw materials to a solvent 50 as to dissolve impurities,removing the solvent with the dissolved impurities therein, applying tothe partially purified materials a reagent of the group consisting of asolution of hydrofiuoric acid and hydro en fluoride to effectdecomposition of silicon compounds present, washing the material withwater to dissolve substantially all the remaining hydrofluoric acid andany flue-silicic compound present and filtering the solution, treatingthe washed material with an alkali sulphide to dissolve polysulphidesand filtering the solution, digesting thecompound with hydrochloric acidto remove acid-soluble impurities and copper sulphides, filtering theresulting solution, washing the relatively pure compound remaining as aresidue, first with water, then an aqueous solution of a volatile alkaliand finally distilled water until a substantially constant electricalconductivity is obtained for the filtrate.

2. The process of producing metallic disulphides, tellurides andselenides having solid lubricant properties from finely divided materialcontaining said compounds which comprises applying to the material anorganic solvent capable of dissolving grease, sulphur and flotationagents, removing the solvent with the dissolved impurities therein,applying to the partially purified material a reagent from the groupconsisting of hydrofluoric acid solutions and hydrogen fluoride toeffect decomposition of any silicon compounds present, washing thefluoride treated material with water to dissolve substantially all thefluorine compound and filtering the solution, applying to the washedmaterial an alkali sulphide solution to dissolve polysulphides andfiltering the solution, heating the residual material at a temperaturesufficient to convert any silicic acid to silica, applying to thematerial a reagent from the group consisting of hydrofluoric acidsolutions and hydrogen fluoride, adding hydrochloric acid, washing thematerial with water and filtering the solution, further digesting thematerial with hydrochloric acid to remove acid-soluble impurities,filtering the solution, washing the material with water, then watercarrying a volatile alkali and finally washing the relatively purecompound so produced with distilled water unti1 a substantial constantelectrical conductivity is obtained for the filtrate.

3. The process of producing metallic disulphides, selenides andtellurides having solid lubricant properties from finely dividedmaterial containing said compounds, comprising subjecting the finelydivided material to an organic solvent capable of dissolving grease andflotation agents, removing the solvent with the dissolved impuritiestherein, applying from 3 to parts of 80% hydrofluoric acid to 8 ports ofthe partially purified compound heating to dryness, repeating thetreatment with the hydrofluoric acidso as to remove volatile im;urities,treating each part or the dried material with from 2 to c parts byweight or hydrochloric acid to remove ecid soluble impurities, filteringand washing the materiel with water to remove the acid, epplying sque0115 ellreli sulphide solution to dissolve metal polysulphides,filtering and washing the residue-ll compound, and washing the compoundin distilled water until a substsntislly constant electricalconductivity is obtained.

4. The process of producing metallic disu1 phides, selenides endtellurides having solid lubrlcent properties from finely dividedmaterial containing said compounds, comprising, finely dividing thematerial to 100 mesh size or finer, errtracting the finely dividedcompound with an organic solvent capable of dissolving grease, flotationagents and sulphur, applying hydrofluoric spouses 5. In the process ofproducin metallic disulphides, selenldes and tellurldes having solidlubricant properties from material containing said compounds whichcomprises, applying to the material organic solvents capable ofdissolving grease and sulphur, removing the organic solvents with theimpurities dissolved therein, applying to the material 9, reagent of thegroup consisting of hydrogen fluoride and solutions of hydrogenfluoride. removing hydrogen fluoride reaction products, washing thematerial with wete d filtering, applying to the residue en aqua solutionof an alkali sulphide to remove polysulphides and filtering, digestingthe meter-lei with hydrochloric sold end filtering and washing toproduce e. solid lubricant residue substantially free from abrasives.

6. In the process of producing metallic disul phides, selenldes andtellurides having solid luhricsnt properties from material containingsaid compounds which comprises, applying to the material organicsolvents capable of dissolving grease and sulphur, removing the organicsolvents with the impurities dissolved therein, milling the material toan extremely fine powder, applying to the finely divided material areagent of the group consisting of hydrogen fluoride and solutions ofhydrogen fluoride, removing hydrogen fluoride reaction products, washingthe material with water and filtering, app-lying to the residue anaqueous solution of an alkali sulphide to remove polysulphides andfiltering, digesting th material with hydrochloric acid and filteringand washing to produce a solid lubricant residue substantially free fromabrasives.

EDWIN B. KAERCHER.

